This invention relates to methods and circuits for triggering electronic instruments, particularly triggering an oscilloscope to display a signal that has multiple potential triggering events during a period of that signal.
An oscilloscope ordinarily has a triggering circuit built. into it that enables the user to view a stable graphic display of a repetitive input signal. A stable display is created by synchronizing the horizontal sweep with the signal that is to be displayed.
Triggering circuits in oscilloscopes typically compare the amplitude of the input signal to a user set amplitude (often referred to as the "trigger level") and start the horizontal sweep signal as soon as the input signal crosses the trigger level in a user selected direction. The reference amplitude often is adjusted so as to start the sweep at a unique point on the repetitive input waveform.
A problem is encountered when trying to use an oscilloscope to display the waveform of a signal that produces multiple triggering events during one sequence of the repetitive input signal. For example, large transients superimposed on a sine wave signal several times during the period of the sine wave may cause the oscilloscope to trigger several times during that period, thereby making it very difficult, if not impossible, to achieve synchronism.
AC and DC motors, and uninterruptable power supplies, often create signals that have multiple triggering events during a single period of the signal. For example, a variable frequency motor controller for an AC motor typically produces a set of sine weighted, pulse width modulated output signals used to drive the AC motor. Since each successive pulse could ordinarily produce a trigger, it is exceedingly difficult to synchronize the oscilloscope sweep signal to the motor controller output waveform.
Another example of multiple triggering events in a single period of a signal can be found in DC motor drive circuits, which produce transient signals on the power line feeding the drive circuit. This happens because the impedance seen by the power line changes when a thyristor turns on to pass power to the motor at some point during each half-cycle. Such transients often propagate to other electrical systems connected to the same power line, such as computer systems, and there produce performance problems. Since the transients ordinarily occur at least twice during one period of the 60 Hz power supply, it is exceedingly difficult to synchronize an oscilloscope to the power line for diagnosing the source of problems for such systems.
Therefore, it can be seen that there is a need for an effective method and circuit for triggering an electronic instrument, such as an oscilloscope, only once during a single period of a signal having multiple triggering events during that period.